LightDM can be configured by directly modifying its configuration script or by using the lightdm-set-defaults applications
that can be found in /usr/lib/lightdm/lightdm/. To see some of the options available, execute:

$ man lightdm-set-defaults

There are, however, a lot more variables to modify in the configuration file than by using the lightdm-set-defaults application.

Changing Background Images/Colors

Users wishing to have a flat color (no image) may simply set the background variable to a hex color.

Example:

background=#000000

If you want to use an image instead, see below.

GTK+ Greeter

Users wishing to customize the wallpaper on the greeter screen need to edit /etc/lightdm/lightdm-gtk-greeter.conf defining the background variable.

Sources of Arch-centric 64x64 Icons

The archlinux-artwork package from the official repositories contains some nice examples that install to /usr/share/archlinux/icons and that can be copied to /usr/share/icons/hicolor/64x64/devices as follows: